The long term objective of this research is to understand how the immune response to the antigens of a primary autochthonous tumor influences the growth of that tumor. We are interested both in influences which operate before the emergence of grossly detectable tumor and those which operate beyond this stage. The former may affect the oncogenic insult itself, pre-malignant stages of the neoplasm or small numbers of malignant cells not yet grossly detectable. The latter must necessarily affect the growth or regression of an established tumor. We are interested both in intrinsic and extrinsic factors which may influence the immunologic relationship between tumor and host. The intrinsic factors include genetic polymorphisms regulating the immune response best studied by comparing animals of different genetic constitution, and changes associated with normal ontogeny which are best studied within an inbred population. Extrinsic influences to be studied will focus on efforts to perturb the immune response. Both specific and non-specific perturbations will be studied at various times relative to application of the oncogenic insult. Attempts will be made to analyse the mechanisms responsible for any observed differences in frequency of tumor development or in tumor growth pattern. The system chosen for this study is that of primary murine Rous sarcoma.